Solar power generation involves collecting solar radiation and converting it to usable energy. Existing technologies for harvesting solar energy include, for example, solar heating and solar photovoltaics. Solar photovoltaics encompass methods of generating electrical power which operate by converting solar radiation into direct current electricity using solar cells made of materials exhibiting the photovoltaic effect. Photovoltaic technology includes concentrating and non-concentrating systems.
Non-concentrating systems include flat panels of photovoltaic solar cells that directly receive solar radiation. In flat panel photovoltaic technology, the solar cells are made of silicon and cover essentially the whole exposed surface of the panels. Current commercial efficiency of silicon-based solar cells is about 16%. Given that silicon has a spectral response limited to the range from 450 to 900 nanometers (nm), commercial expectation of conventional photovoltaic technology does not exceed about 20%.
Concentrated photovoltaic (CPV) systems use solar concentrators made of lenses, mirrors or other optical components to concentrate large amounts of solar radiation onto a small area of photovoltaic solar cells to generate electricity. For example, high concentration photovoltaic (HCPV) systems use solar concentrators that can concentrate sunlight to intensities of 100 and 2500 suns or more onto a multi-junction solar cell (1 sun=1 kW/m2). In solar concentrators, solar radiation may undergo various states of refraction and reflection before reaching the multi-junction solar cells. Compared to non-concentrated photovoltaic systems, the solar cell elements in CPV systems may be made more compact. As a result, the amount of photovoltaic material, which is generally one of the most expensive elements in solar power generation systems, can be reduced and costs can be lowered.
The efficiency of a solar concentrator depends not only on the amount of solar energy captured by the solar concentrator, but also on its ability to accurately direct the concentrated sunlight onto solar cells having a relatively small photovoltaic area. In contrast to their non-concentrating counterparts, CPV systems generally need to remain constantly aligned with the direct normal incidence (DNI) solar radiation, which is collimated at an angle of approximately ±0.27 degree. Improper alignment can cause a significant portion of the available energy to be lost. In order to maintain proper alignment, CPV modules are typically mounted on high-precision tracking systems (e.g., with precision of the order of ±0.1 to ±0.2 degree) that track the sun as it moves across the sky so as to maximize exposure to and collection of the DNI solar radiation.
The tolerance of a solar concentrator to misalignment with respect to DNI solar radiation may be characterized by the “acceptance angle” of the solar concentrator. In the CPV field, the acceptance angle is often defined as the angle of incidence of solar radiation at which the energy losses of the concentrator are increased by a certain amount, generally 10%, compared to the energy losses at DNI. The acceptance angle of a solar concentrator varies inversely with its concentration factor such that for a given acceptance angle, there exists a maximum theoretical concentration factor that cannot be exceeded. However, although currently used HCPV systems can achieve a wide range of concentration factors, their acceptance angles are generally limited to ±0.5 to ±0.8 degree.
As mentioned above, HCPV systems typically use multi-junction solar cells based on III-V semiconductors rather than silicon-based solar cells. Multi-junction solar cells generally include three layers, each layer being tuned to extract energy in a specific wavelength band of the solar spectrum. As a result, triple-junction cells can exhibit better spectral response than silicon-based cells, with an energy conversion efficiency of sunlight into electricity expected to be around 55%. Commercial efficiency of currently available triple-junctions cells is about 40-44%.
Solar concentrators for use in CPV modules operate by focusing DNI solar radiation to the photovoltaic solar cells. Solar concentrators commonly use a primary optical element and a secondary optical element. Incident solar radiation is first focused by the primary optical element for concentrating solar power. The concentrated solar radiation is directed toward the secondary optical element, which can provide homogenization and, optionally, further concentration. Each of the primary and secondary optical elements may include refractive, reflective and diffractive optics.
In solar concentrators used in HCPV modules, the overall concentration factor is usually provided mainly by the primary optical element, while the secondary optical element is used mainly for homogenization purposes. More specifically, the role of the secondary optical element is mainly to distribute the solar radiation concentrated by the primary optical element uniformly across the photovoltaic area of the solar cell. The homogenization provided by the secondary optical element contributes to enhancing energy conversion efficiency and mitigating the risk of forming “hot spots” which could lead to poor fill factors, to cell damage or to failure.
In refractive-type solar concentrators, Fresnel lenses are generally used as a primary optical element because of their low manufacturing costs. However, Fresnel lenses exhibit optical transmission coefficients limited to between about 80 and 85%. As result, HCPV systems based on Fresnel lenses and triple-junction solar cells would be expected to exhibit a peak efficiency of about 32% (i.e., 40%×80%), which is about twice the overall efficiency of non-concentrated photovoltaic technology. However, the effective efficiency (i.e., the actual kWh generated) of commercial HCPV systems generally does not exceed 23%. Attempts to increase the efficiency of solar concentrators have included using better tracking systems to maximize the intensity of incoming radiation, and modifying the materials entering the fabrication of optical components to enhance their reflective and refractive properties. The efficiency of HCPV solar contractors has also been addressed by changing the design of their individual components in view of increasing the acceptance angle.
In this context, a recent study [B. Stafford et al., “Tracker accuracy: field experience, analysis, and correlation with meteorological conditions” Photovoltaic Specialists Conference (PVSC), 2009 34th IEEE, p. 002256-002259, 7-12 Jun. 2009] has measured the performances of real HCPV systems installed in the field, and has highlighted the importance of the acceptance angle in their overall efficiency. The study found that HCPV modules having acceptance angles of ±0.5 degree and ±1 degree would respectively generate 60% and 25% of additional losses, even when using high-precision tracking systems with precision of the order of ±0.1 to ±0.2 degree. In other words, because of these additional losses, HCPV systems with low acceptance angles will tend to lose their alignment with DNI solar radiation more easily, thus collecting and generating significantly less energy than predicted. These additional losses can arise from manufacturing defects and misalignment as well as from deformations caused by external influences such as, for example, wind, energy fluctuations between DNI and circumsolar solar radiations, long term aging, temperature, gravity, mechanical stress, and the like.
In light of the above, it will be understood that solar concentrators having large acceptance angle can increase the efficiency of CPV systems, which, in turn, can decrease the cost-per-kWh and help drive solar power generation toward grid parity.
There therefore remains a need in the art for solar concentrators having improved acceptance angles while maintaining high concentration factors and optical efficiency.